1. Field of the Invention
This invention relates to a three-position working cylinder having dual concentric pistons and a piston rod extending from the pistons for attachment to an external device to be controlled to such three positions, such an external device possibly being a ventilation flap on a passenger vehicle. Typical three-position working cylinders provide an auxiliary piston disposed on a primary center piston.
2. Brief Description of the Prior Art
An example of such an approach can be found in German Pat. No. DE3107908, which includes a cylinder having a first piston disposed therein such that the cylinder is divided into a first pressure chamber on one side of the first piston, and a second pressure chamber located on the opposite side of the first piston. A piston rod extends from the first piston and a second piston designed as an annular piston is disposed on the first piston, and can be moved in a sealed manner. The second piston is moved against the graduation of the cylinder housing, which defines a middle position. Two stops provided on the first piston serve to limit the relative movement between the two pistons in such a manner that the second piston can be moved between the stops relative to the first piston. An end stop, located in the end region of the first piston when the first piston is activated by means of the piston rod, prevents the second piston from coming loose from the first piston. This end stop, similar to the second stop, is designed as a circlip ring. The disadvantage of such an approach is that, during the assembly of this work cylinder, first the second piston must be pushed onto the first piston, and then the circlip rings must be installed for the above-mentioned end stop. Another known three-position work cylinder is German Pat. No. DE1576175, which provides that, when the two pistons are in motion, a vent opening is provided in the cylinder wall leading to the atmosphere from the chamber formed between the two pistons. Such chamber and vent opening arrangement are primarily designed to prevent the formation of an impact pressure or a vacuum in the space between the two pistons. Such a vent opening has the disadvantage that moisture and dirt can penetrate the cylinder. Even if an attempt were made to prevent the penetration of moisture and dirt by placing a check valve at the vent opening to atmosphere, then the disadvantage of a vacuum formation would again result.